Rapidly dividing neoplastic cells characteristically have high metabolic rates, are O2 limited, and generate excess acid. Thus, physiological indices such as PO2, pH, and excess lactate are often targeted for detection and quantification as an index of tumor activity. Drugs that target cancer cells are often designed to take advantage of the unusually large pH gradient across the plasma membrane of neoplastic tissues, so knowing the extracellular pH status of a tumor prior to treatment would be extremely useful. Unfortunately, there is no simple, non-invasive method available to the clinician to monitor this biological indicator of cell activity. Numerous exogenous pH sensitive 'indicators' have been devised and demonstrated in animal models, yet none have so far have proven clinically useful in characterizing tumors in human studies. Our long-term objective is to develop pH and redox sensitive agents that report these biological indices by using the magnetic resonance (MR) signal of bulk water as an antenna. A new ligand was recently discovered that binds gadolinium in a macrocyclic cavity and protects a single metal ion-bound water molecule from exchanging rapidly with bulk water. This feature, historically considered detrimental to MR contrast agent design, appears to have beneficial characteristics for making biologically responsive or functional agents. The purpose of this grant is to explore the extent to which this property can be utilized in the design of agents that report pH, redox, and perhaps other biological indicators of cell activity directly in a MR image. Two classes of agents will be examined. In the first, proton exchange between a slowly exchanging gadolinium-bound water molecule and bulk water has been shown to be pH dependent in a model system. One goal here is to fine tune that system to generate the most efficient (high relaxivity), pH sensitive, contrast agent. A second goal is to use other lanthanide ions having smaller magnetic moments to prepare systems whereby magnetization is transferred via selective, single frequency, presaturation of the bound water resonance. This new type of MR contrast agent offers the promise of a direct quantitative measure of pH or cell redox without a separate measurement of agent concentration. If successful, both types of agents could be readily applied in human cancer detection and characterization using conventional, clinical MR imaging equipment.